Production of transfer images by the silver salt diffusion process



llnitd This invention relates to the production of transfer images by the silver salt diffusion process.

In French Patent No. 879,995 there is described a photographic process for the direct production of positive images, hereinafter referred to as silver salt diffusion processji according to which a light-sensitive silver halide emulsion layer, hereinafter referred to as negative layer, after being exposed to an object is developed in the presence of a silver halide solvent, being in close contact during said developing process with a light-insensitive reception layer, hereinafter referred to as transfer layer, containing nuclei for development, or substances producing such nuclei for development. During this process the exposed part of the silver halide emulsion layer is developed to a negative image of the object to which it had been exposed while at the same time part of the unexposed silver halide is dissolved by the silver halide solvent. This dissolved silver halide diffuses to a transfer layer, where it is reduced to a positive image by the developer under the catalytic action of the nuclei for development present in the said layer. The meaning of the terms negative and positive as used in this description is relative, which means that if the emulsion layer is exposed to a negative original, a direct negative will be produced in the reception layer whereas if the emulsion layer is exposed to a positive original a direct positive will be obtained in the said transfer layer.

According to a modification of the aforementioned process, it is also possible to produce several prints from one negative.

A modification of the aforementioned process according to which several transfer prints are produced from a single silver halide emulsion layer is described in Us. Patents Nos. 2,725,298 and 2,843,485. According to these patcuts a photographic element is provided comprising a silver halide emulsion layer and having coated thereon a gelatin or polyvinyl alcohol layer containing physical development nuclei, for example colloidal silver or chemical precipitants for silver ions. If this element is treated after exposure with a silver halide developer containing a silver halide solvent, there is obtained in the silver halide emulsion (negative) layer a negative silver image and in the upper layer a positive silver image of the copied original. The upper layer is then transferred in several continuous portions onto absorbent supports to produce several positives. It is, however, in many cases diflicult to detach the positive layer in several strata from the negative layer due to the strong cohesion of the former as a result of which this layer tends to be completely detached from the negative layer with the first print.

it has now been found that these disadvantages are obviated if binding agents are used for the positive layer which possess only a comparatively weak cohesion and to which are added organic compounds that do not form continuous films and which are readily soluble in Water and alkalis. In this way, positive layers are obtained which can easily be pulled off in several thin successive strata. As binding agents, there may be employed watersoluble starch ethers or carboxymethyl cellulose. Suitable starch ethers are the methyl, ethyl, and starch ethers containing per anhydroglucose unit a carboxymethoxy o-cH,cooH

rates atent O 3,067,033 Patented Dec. 4, 1962 o'r hydroxyethoxy (-OCH CH OH) radical, whereby the carboxyrnethyl ether may be used in the form of its alkali metal, ammonium or other water-soluble salts. Polyethylene glycols having a molecular weight of about 200 to 2000 are particularly suitable organic compounds having no film-forming capacity. These polyethylene glycols substantially facilitate the layer being printed olf in thin portions and are unable to penetrate to an appreciable degree into the negative layer. They are preferably used in amounts of 10-70 percent by weight as calculated on the total amount of binding agent plus polyethylene glycol.

The layers produced from these binding agents when immersed in a salt-containing solution, such as a silver halide developer solution, have little swelling capacity and adhere firmly to the negative layer, whereas they swell very strongly when immersed in ordinary water, so that their layer cohesion is lost and they can easily be separated from the negative layer.

As is customary in the silver salt diffusion process, physical development nuclei, for example, colloidal heavy metals, such as colloidal silver, heavy metal sulphides or heavy metal selenides such as silver, lead, zinc, and cadmium sulfides, may be added to the positive layer in order to produce a sufliciently rapid and copious deposition of silver in the positive layer. It is also preferable to add to these layers compounds such as heterocyclic nitrogensulphur compounds, for example 5-phenylmercaptotetrazole, which on the one hand lead to a black form of silver deposit and on the other hand produce pure whites.

These materials can be processed in various ways. For example, after exposure of the negative layer, the material may be conducted through a silver halide developer solution containing a silver halide solvent. The developer and the silver halide solvent, for example sodium, potassium, or ammonium thiosulfate, penetrate through the positive layer into the negative layer. In the latter, the exposed silver halide is developed and the unexposed silver halide is dissolved. The dissolved silver halide diffuses into the positive layer and forms therein the positive of the copied original.

In order to produce several positives itis possible to proceed as follows:

(1) A sheet of ordinary writing paper is placed on the positive layer, which is still in close contact with the negative layer, and these combined sheets are passed through a tray or tank of water. The excess water is squeezed out by a roller wringer or by a pair of rollers and the writing paper is separated from the positive layer. Either the entire positive layer or only the uppermost stratum thereof adheres to the paper, depending on the composition of the positive layer and its thickness. The positive layer can be detached as a single layer if it is from '1 to 3 microns thick. With greater layer thicknesses, for example of from 5 to 15 microns, a larger number of layers or strata can be detached. When the uppermost strata of the positive layer have been detached, the process can be repeated several times until the entire positive layer has been removed from the negative layer. In this way, several positive prints are obtained from one negative.

If layers are employed which contain a comparatively large quantity of polyethylene glycol, they soften very considerably in a developer which contains a salt. The positive layer can then be transferred from the negative layer to another support during development, either all at once or stratum by stratum.

(2) For the processing of the two-layer material, it is also possible to use a developer solution which does not contain any silver halide solvent. The two-layer material, after brief initial development, is brought into contact with a water-pervious colloid layer containing a silver (a halide solvent. This process has the advantage that the negative is first developed before the silver halide solvent can become eifective. If nuclei are present in the layer containing the silver halide solvent, a positive image of the copied original is obtained, both in the positive layer of the two-layer material and in the layer containing the silver halide solvent. After the two-layer material has been separated from the layer with the silver halide solvent, the positive layer of the two-layer material can be transferred to another support, for example to a support consisting of normal writing paper, in the manner described above and either all at once or stratum by stratum. If it is desired to produce a large number of prints from the negative-positive layer system, it is advisable to apply the positive layer in two separate working steps, for example, by choosing as bottom layer a material which swells to a lesser degree in water than the upper layer. The silver content of the negative layer may be varied according to the other conditions which are chosen. Layers containing less than 1 g. of silver per square meter usually only produce from 2 to 3 positives, whereas layers containing from 3 to 4 g. of silver produce substantially more positives.

Hardened gelatin is generally employed as binding agent for the negative layer. Substances which facilitate detachment of the positive layer, such as an emulsion of stearic acid, may be added to the gelatin layer if desired.

In each case the sharpness of the positives obtained is satisfactory, the positives being sharper the further they are removed from the negative layer when they are formed. The precipitation of the dissolved silver salts on the development nuclei requires a certain high silver salt concentration the range of which is always narrower with increasing distance from the negative because of a lateral diffusion. A white (i.e. unexposed) line in the negative produces a line which becomes progressively thinner as the positive is formed further from the negative, which is recorded by the eye as an increase in sharpness. It has also been found that the silver precipitation in the positive layer occurs quite uniformly throughout the entire cross section of the layer if the choice and quantity of the nuclei are correctly measured.

It is of course also possible to process material mechanically in the apparatus which have been provided for processing papers by the silver salt diffusion process. It is possible in this case to provide a supplementary device in which only water is used instead of developer solution and in which the developed positive layer is brought into contact with writing paper or other absorbent sheet material.

Example 1 100 cc. of a 5 percent stearic acid emulsion are added to a liter of an ordinary silver chloride emulsion together with clarifying, hardening and wetting agents. This emulsion is used to form a negative layer containing 1.5 g. of silver per square meter.

A positive layer is coated on to the negative layer. The positive layer is cast from a solution having the following composition:

1000 cc. of water g. of the sodium salt of carboxymethyl cellulose g. of polyethylene glycol (molecular weight about 0.01 g. of colloidal silver sulfide 0.05 g. of 5-phenylmercaptotetrazole, as well as wetting agent, such as saponin, in order to obtain a satisfactory casting For processing purposes, the following developer solution is used:

12 g. of caustic soda 12 g. of sodium thiosulphate The processing takes place in the apparatus customarily employed in silver salt diffusion processes.

The two-layer material after being exposed to the original to be reproduced is first passed by itself through the developer solution. A sheet of typewriting paper is then laid on the positive layer, whereafter the material while in contact with the said paper is passed through the apparatus once more. Then typewriting paper is then separated from the two-layer material.

A major part of the positive layer remains adhered to the typewriting paper. By repeating the operation, 2 to 3 more positive copies can be produced.

Example 2 An ordinary gelatino silver chloride emulsion containing a stabilizer, such as benzotriazole in usual amounts, is cast onto a paper or film support to produce an emulsion layer containing 2.5 g. of silver per square meter. A

econd layer is applied to the aforementioned layer. The second layer is cast from a solution having the following composition:

1000 cc. of Water 30 g. of starch carboxy methyl ether 15 g. of polyethylene glycol (molecular weight about 0.05 g. of colloidal silver 0.06 g. of 5-phenylmercaptotetrazole Also included in tihs composition are conventional casting additives to ensure a good wetting of the first casting.

The thickness of the layer should be 12 microns when dry.

After exposure to light, this 2-layer material is developed at room temperature for about 1 minute in a developer having the composition:

1000 cc. of water 15 g. of hydroquinone 0.5 g. of l-phenyl-3-pyrazolidone g. of anhydrous sodium sulfite 10 g. of caustic soda 1 g. of potassium bromide The development is carried through in a convential apparatus used for carrying out the silver salt difiusion process. The two-layer material is then introduced through one of the slots and an additional positive layer containing physical development nuclei and a silver halide solvent that was coated on a separate paper support is introduced into another slot.

This latter layer is cast from a solution having the following composition:

1000 cc. of water 0.3 g. of citric acid 60 g. of gelatin 0.01 g. of colloidal silver sulfide 8 cc. of 40 percent formalin solution 15 g. of sodium thiosulfate After the papers have passed through the developer, they are kept in contact for about 2 minutes and then they are pulled apart. A laterally unreversed positive is observed on the layer containing the silver halide solvent, while a laterally inverted positive is seen on the positive layer, which is still adhering firmly to the negative layer.

Detachment of the positive layer stratum by stratum from the negative layer is effected in this case by means of water. A sheet of writing paper is placed on the twolayer material and the sheets are passed in contact through a tray or tank of water the excess water being squeezed out by rollers. The Writing paper is detached from the two-layer'material and it is seen that the uppermost part of the positive layer adheres to the writing paper. This operation can be repeated about times, that is to say, about 10 positive prints can be obtained from one negative.

What is claimed is:

1. A process for the production of photographic transfer images by the silver salt diifusion process which comprises exposing a photographic sheet material having a light-sensitive silver halide emulsion layer and coated thereon an unsensitized colloid layer, the colloid of which consists essentially of a member of the group consisting of a water-soluble starch ether and carboxymethyl cellulose, said colloid layer having incorporated therein a polyethylene glycol and physical development nuclei, developing the said exposed photographic material in a silver halide developing solution in the presence of a silver halide solvent to produce a negative silver image in the said silver halide emulsion layer and a positive silver image in the said colloid layer, and transferring at least a stratum of the said colloid layer containing the said positive silver image onto a separate support.

2. A process according to claim 1, wherein the said exposed photographic material is developed in a silver halide developer solution while the colloid layer of the said photographic material is kept in contact with a lightinsensitive water-permeable colloid layer coated on a separate support, the said water-permeable colloid layer containing a silver halide solvent and physical development nuclei, which last mentioned silver halide solvent diffuses during development into the said exposed photographic material.

3. A process according to claim 1, wherein the said transfer of the said colloidal layer containing the said positive silver image onto a separate support is carried through after the said developer solution has been at least partially washed out from the said colloid layer.

References Cited in the file of this patent UNITED STATES PATENTS 2,423,549 Blake et a1 July 8, 1947 2,531,832 Stanton Nov. 28, 1950 2,572,357 Land Oct. 23, 1951 2,662,822 Land Dec. 15, 1953 2,725,298 Yutzy et al Nov. 29, 1955 2,819,662 Land Jan. 14, 1958 FOREIGN PATENTS 706,333 Great Britain Mar. 31, 1954 

1. A PROCESS FOR THE PRODUCTION OF PHOTOGRAPHIC TRANSFER IMAGES BY THE SILVER SALT DIFFUSION PROCESS WHICH COMPRISES EXPOSING A PHOTOGRAPHIC SHEET MATERIAL HAVING A LIGHT-SENSITIVE SILVER HALIDE EMULSION LAYER AND COATED THEREON AN UNSENSITIZED COLLOID LAYER, THE COLLOID OF WHICH CONSISTS ESSENTIALLY OF A MEMBER OF THE GROUP CONSISTING OF A WATER-SOLUBLE STARCH ETHER AND CARBOXYMETHYL CELLULOSE, SAID COLLOID LAYER HAVING INCORPORATED THEREIN A POLYETHYLENE GLYCOL AND PHYSICAL DEVELOPMENT NUCLEI, DEVELOPING THE SAID EXPOSED PHOTOGRAPHIC MATERIAL IN A SILVER HALIDE DEVELOPING SOLUTION IN THE PRESENCE OF A SILVER HALIDE SOLVENT TO PRODUCE A NEGATIVE SILVER IMAGE IN THE SAID SILVER HALIDE EMULSION LAYER AND A POSITIVE SILVER IMAGE IN THE SAID COLLOID LAYER, AND TRANSFERRING AT LEAST A STRATUM OF THE SAID COLLOID LAYER CONTAINING THE SAID POSITIVE SILVER IMAGE ONTO A SEPARATE SUPPORT. 